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you seem to think 400 IU of vitamin D is adequate, it isn't. If some so-called expert says this is so, they don't know what they are talkiing about. It has long been know that a person living in the tropics gets far higher levels of vitamin D from synthesis due sunlight in the area of 4000 IUs per day. Ten times 400!!! Since you are low in vitamin D you aren't getting enough sun. Aging, sunscreen, and frequent bathing reduces vitamin D synthesis at the skin, as I recall. More recent research has shown low level supplementations with 400 IU (10 mcgs) of vitamin D to often fails to increase subject status to what is consider an adequate level. Even the quite conservative Food and Nutrition Board of the Institute of Medicine has set the tolerable upper limit at 2000 IUs for CHILDREN, adults, pregnant and lactating women. So it is clear 1000 IU isn't going to hurt. During WINTER MONTHS, I've chosen to take 4000 IUs of vitamin D3. And yes that is twice the UL. Clearly I think the actual real world UL is higher. I wouldn't do that with vitamin D2 as this form of D is found only at rather low levels when compared to D3 in nature. If you are conservative, I'll suggest the supplement dose size of 800 IU is a bare minamum and 1000 IUs a tad more reasonable in the context of the use of other supplements for YEAR AROUND USE.
Please recall that corticosteroids are said to impair vitamin D metabolism. Moreover, corticosteroids reduces calcium absorption.
Your other question as to the fate of vitamin D in the body is an interesting one. Vitamin D doesn't seem to have as long a potential storage life a vitamin A, albeit a longer than vitamin K and E. By the way E biological half life is rather short.....think days. I am running on memory in the previous two sentences.
Now back to your question, there was a recent report of a study in which the subjects took one 100,000 IUs vitamin D3 dose every three months. Which averages to 800 IUs per day. Albeit the researcher's bottom line recommend was quite cowardly IMO as he recommended vitamin D in the range of 400 to 800 per day. His subject taking the vitamin D showed a marked decrease in fractures. Kay Tee Khaw was lead researcher. There is evidence that 400 IU per day supplements are too small. And remember he used an average of 800 IUs per day plus a calcium supplement of 1200 mgs.
Recall, that vitamin D is regulated by activation of the "vitamin" form into various active forms. First, the vitamin is modified by the liver into 25-(OH)D. This form is still not an active form but is just an inactive substrate. However, the levels of 25-(OH)D found in tests do provide a good measure of vitamin D status. Renal synthesis of 1,25-(OH)2D is quite tightly controlled. This active form regulates bone mineral homeostasis by a number of avenues. Intestinal epithelial cells have a receptor type to which the active form binds. This stimulation combined with retinoid action on another receptor results in the production of calbindin a calcium binding protein the with plays a role in pump Ca++ out of cells. The active form effects the microvilli so as to increase the permeability of calcium ions thru their membranes. It thus helps getting Ca++ in from intestinal lumen. In the bone, the osteoprogenitor cells and mature osteoblasts all contain 1,25-(OH)2D receptors. Their activation by the activated vitamin D results in the differentiation of these cells. Here the actions get fairly complicated. There is an involvement with bone reabsorption, lysomal enzyme release, and reduced collagen synthesis in the bone according to in vitro research. In the kidney this active form has actions like those in the intestine that serve to promote the recovery of Ca++ from the dilute out going liquid. Futher it must emphasized that the activated form of D regulates growth and differentiation many tissue cells. In other words, it plays a role in the prevention of skin, prostate, and breast cancer!!! It plays a role in the regulation of PTH. It is involved in insulin secretion. And the immune system are in part modulated by 1,25-(OH)2 D. In test animals it served to both increase protective responses to infection and suppresses autoimmune damage. There is still much research needing to be done in humans. The lack of fully developed research reflects a full blown failure on the part our science system, IMO.
In a related topic, it is my opinion a person should be taking a vitamin K supplement that is above 1 milligram per day and not some silly little 100 microgram dose. Or at the very least choosing eat greens cooked with a modest amount of oil or fat to enhance vitamin K. Note also that vitamin K helps to prevent soft tissue calcinosis/calcification. Calcinosis is the lasting damage that huge doses of vitamin D has caused. Remember vitamin K is as important as D to bone health!
I suggest you do a Google search of some of the other Newsgroup forums such as sci.med.nutrition, sci.med, sci.life-extension on the topic of vitamin D.
I left loads out and there are points I am unclear on myself. I'll suggest since you are the moderator that you start a hard copy notebooks a number of common bone metabolism issues such as vitamin D, vitamin K, calcium, other minerals, the various forms of hormone replacement therapies used in men and women, common aging changes that makes persons prone to osteopenia and osteoporosis, alternative remedies and ideas, and current standard medical protocols. What is that 10 notebooks? That is a bit much. And yet even I a would be denizen of the life-extension forum has that many computer sub-directors dedicated to these topics. Some selective cross-posting maybe in order so all in THIS forum get to see the better responses from the other groups. If you can bump out the trolls it would be nice.
Warning!! I do not claim to be a medical professional...... This is serious. Trust but verify.....better...'Verify then trust'
Finally I'll try to provide references if you require it on a specific point.
William A. Noyes
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